Get --enable-gcc-warnings working again.
The recent changes to configure.ac removed the transliteration of -I to -isystem in CFLAGS, which is needed for --enable-gcc-warnings. Bring this back while keeping the spirit of the recent changes. * configure.ac (edit_cflags): Restore this shell var, and put it at the top level, where it'll be useful when emacs-24 is next merged. (EMACS_CHECK_MODULES): New macro. All uses of PKG_CHECK_MODULES changed to use it.
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